HIV Fusion Peptide Penetrates, Disorders and Softens T-Cell Membrane Mimics

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This work investigates the interaction of N-terminal gp41 fusion peptide (FP) of HIV-1 virus with model membranes in order to elucidate how FP leads to fusion of HIV and T-cell membranes. FP constructs were (i) wildtype FP23 (23 N-terminal amino acids of gp41), (ii) water soluble monomeric FP that adds six lysines on the C-terminus of FP23 (FPwsm) and (iii) the C-terminus covalently linked trimeric version (FPtri) of FPwsm. Model membranes were (i) LM3 (a T-cell mimic), (ii) DOPC, (iii) DOPC/30mole%cholesterol, (iv) diC22:1PC and (v) diC22:1PC/ 30mole%cholesterol. Diffuse synchrotron low-angle x-ray scattering (LAXS) from fully hydrated samples, supplemented by volumetric data, showed that FP23 and FPtri penetrate into the hydrocarbon region and cause membranes to thin. Depth of penetration appears to depend upon a complex combination of factors including bilayer thickness, presence of cholesterol and electrostatics. X-ray data showed an increase in curvature in hexagonal phase DOPE which further indicates that FP23 penetrates into the hydrocarbon region rather than residing in the interfacial headgroup region. LAXS data also yielded bending moduli KC, a measure of membrane stiffness, and wide-angle x-ray scattering (WAXS) yielded the Sxray orientational order parameter. Both FP23 and FPtri decreased KC and Sxray considerably, while the weak effect of FPwsm suggests that it did not partition strongly into LM3 model membranes. Our results are consistent with the HIV fusion peptide disordering and softening the T-cell membrane, thereby lowering the activation energy for viral membrane fusion. This research was supported by NIH Grant GM 44976 (STN,RC,JFN) and NIH AI 47153(WQ,DPW).